1. Technical Field of the Invention
The present invention relates in general, to power conversion and, more specifically, to a multiple output flyback converter, having improved cross-regulation and soft-switching to eliminate switching losses in the converter.
2. Description of the Related Art
Power converters are widely used to provide required voltages and load currents especially in environments that have higher concentrations of telecommunications or computer equipment. These power converters are often required to provide higher levels of output power for a small physical volume, since space is often at a premium. This requirement dictates that the power converters be high power density devices. These higher power density concentrations also dictate that the power converter operate with as high an efficiency as possible to minimize the converter""s heat generation and therefore its operating temperature rise to the degree possible.
Through the use of switching regulator techniques, power converter efficiencies may be achieved that are typically higher than those of linear regulation techniques. These switching regulator techniques, however, give rise to other efficiency-decreasing problems that are not typically encountered in linear regulators. Because of higher switching frequencies (50 to 100 kilohertz), parasitic circuit inductance and capacitance elements in the transformers and switching devices cause xe2x80x9cringingxe2x80x9d due to circuit resonances. This ringing typically presents both device and efficiency problems that must be addressed to operate the converters at high power densities.
One prior-art method for increasing power density of dc-to-dc converters, particularly in applications where more than one output voltage is required, is to use a single power stage with multiple windings on the power transformer, one for each output voltage. In such a converter, all outputs share the same inverter stage, and only one output, called the main output, is fully regulated by pulse-width modulation of the inverter switches on the primary side. Such a solution then requires feedback isolation, and additional post-regulators are required for independent regulation of the auxiliary outputs against load variation.
The flyback converter is a very popular power supply topology for use in low-power,multiple output applications. When the main output voltage regulation requirement is moderate, these converters may be operated with the main output being regulated by the switching regulator and the other output (s) being xe2x80x9ccross-regulatedxe2x80x9d, i.e., the ratio of the output voltages on the different windings is determined by the winding ratio and the actual relative voltage levels are determined by the xe2x80x98onxe2x80x99 time of the main switch. Cross-regulation of each output voltage is achieved by the turns ratio of the secondary windings.
FIG. 1 illustrates a prior art flyback converter that uses a single switch on the primary side of the transformer to directly regulate one of the output voltage, i.e., Va, using feedback control. The cross-regulation of the other output voltages, i.e., Vb and Vc, is achieved by the turns ratio of the secondary winding. A drawback of the flyback converter of FIG. 1 is the presence of parasitics which result in wide tolerances for the cross-regulated outputs, which do not satisfy strict regulation requirements. Post-regulators (e.g., linear regulators) are sometimes used to improve the regulation of the output voltages. However, this approach increases cost and reduces efficiency of the power supply. A secondary side control method has been proposed as a solution to this shortcoming.
European patent No. 0 698 959 discloses one secondary-side control method which includes switches on the secondary-side of the transformer to provide independently regulated outputs. FIG. 2 illustrates a flyback converter where a rectifying diode is connected in series with each secondary winding. The corresponding regulator RS1 of the first output circuit controls the regulator (RS0) of the primary-side circuit. Output regulators RS2, RS3, control the secondary-side switches S2, S3 correspondingly. The input DC voltage Vin, a voltage representation of the primary-side switch S0 and one output of the regulators RS2, RS3 are also fed to the primary-side regulator RS0.
EP 0 772 284 discloses another secondary-side control method where one secondary winding supplies multiple output voltages via separate branches as shown in FIG. 3. Each output voltage is rectified by means of a diode. The first output V1 is regulated by the pulse width of the primary-sie switch S0. A switch is inserted into each branch supplying the rest of the output voltages (V2, V3). A regulator senses each of the output voltages V2, V3. The regulator controls the duty cycle of the corresponding switch to regulate the output voltage.
U.S. Pat. No. 5,617,015 to Goder et al., discloses a voltage regulator providing multiple independently regulated outputs. FIG. 4 illustrates a boost topology circuit configuration disclosed in Goder. However, it is disclosed that voltage-regulator may be implemented with a flyback or other topologies. A dedicated switch to each output voltage controls the energy delivery to the output. Energy is delivered to only those outputs that have fallen below the lower limit of an acceptable range. The voltage regulator will stop supplying the outputs that have exceeded the upper limit of an acceptable range.
European patents Nos. 0 698 959 and 0 772 284 and U.S. Pat. No. 5,617,015 to Goder each recite control of the on/off state of the primary-side switch based on the values of the output voltages. While each resolve the problem of primary side control of one of the outputs, they share a common drawback. Specifically, the secondary side control methods disclosed by each result in hard switching of the primary-side switch thus increasing switching losses and thereby reducing the efficiency of the power converter.
A further drawback associated with the circuits of FIG. 2 and 3 is that one of the output voltages has no secondary-side switch dedicated to its regulation. Instead, the output includes a rectifying diode and is regulated by the primary side switch. This is problematic in reduced power modes of operation (e.g., standby) where it is desirable to disable some of the outputs. This cannot be achieved with the circuit configurations of FIGS. 2 and 3 because the output which does not include the secondary-side switch cannot be disabled without disabling all of the outputs.
A still further drawback associated with the circuit of FIG. 4 is that the regulation method described therein increases the peak output current resulting in increased ripple compared with methods that continuously provide current to the multiple outputs.
In order to solve problems associated with parasitics which result in wide tolerances for the cross-regulated outputs, an objective of this invention is to make the converter insensitive to these parasitic effects by providing secondary-side control.
A related objective of the present invention is to enable soft-switching of the primary side switch by taking advantage of the method of secondary-side control.
To address the above-discussed deficiencies of the prior art, the present invention provides, a circuit which combines secondary-side regulation with primary-side zero-volt switching to achieve a level of circuit efficiency and flexibility not available in prior art circuit constructions.
The novel circuit provides circuit efficiency by providing zero-volt switching of the primary side switch. Zero-volt switching is achieved by taking advantage of a resonance which occurs between a switch capacitor coupled in parallel with a primary side power switch and a magnetization inductance of a primary side winding. A primary side controller detects the resonance condition and switches the power switch at one of the resonant waveform minima.
The novel circuit further provides flexibility via secondary-side regulation. In particular, the circuit provides means for independently and selectively disabling one or more output channels of the converter by providing a secondary side switch in each of a plurality of output circuits. An output circuit or channel can be selectively disabled, via the switch, by blocking the secondary winding current from flowing into the selected output.
In an embodiment to be illustrated and described, the isolation power converter is a flyback isolation converter including a power transformer, a power switch on the primary side and a secondary side including three output channels connected to a single secondary winding, where the first two output channels switched in each energy cycle are regulated on the secondary side and the last output channel to be switched in each energy cycle is controlled from the primary side. Primary-side control of the last output to be switched in the sequence is effected by an error signal which is transmitted from the secondary-side, via an opto-coupler, to a pulse-width modulator on the primary side for controlling the duty cycle of the power switch on the primary side. While separate voltage controlled switches are include in each output channel, the switch associated with the final output channel to be sequenced is subsidiary to the primary side power switch. However, separate switches are included in each output channel to selectively disable one or more of the output channels by blocking the secondary winding current from flowing into the disabled output(s).
The present invention introduces, in another aspect, a circuit capable of effecting substantially zero-volt switching of a power switch of the primary switching circuit in the primary side switch to increase the efficiency of the power converter.
In an embodiment to be illustrated and described, the primary and secondary side switches are metal oxide semiconductor field-effect transistor switches (MOSFETs). Those skilled in the pertinent art will understand, however, that the present invention fully encompasses all controllable switches, whether conventional or later-developed. An external diode may be employed as required to supplement the internal body diode of the clamping switch. Of course, an external diode may be employed if the clamping switch does not include a body diode.
In one embodiment of the present invention, the converter is selected from the group consisting of a flyback converter, a forward converter and a half-bridge converter. Those skilled in the pertinent art understand, however, that other isolated converter topologies are well within the broad scope of the present invention.